Gallium nitride materials and methods associated with the same

ABSTRACT

Semiconductor materials including a gallium nitride material region and methods associated with such structures are provided. The semiconductor structures include a strain-absorbing layer formed within the structure. The strain-absorbing layer may be formed between the substrate (e.g., a silicon substrate) and an overlying layer. It may be preferable for the strain-absorbing layer to be very thin, have an amorphous structure and be formed of a silicon nitride-based material. The strain-absorbing layer may reduce the number of misfit dislocations formed in the overlying layer (e.g., a nitride-based material layer) which limits formation of other types of defects in other overlying layers (e.g., gallium nitride material region), amongst other advantages. Thus, the presence of the strain-absorbing layer may improve the quality of the gallium nitride material region which can lead to improved device performance.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/096,505, filed Apr. 1, 2005, entitled “Gallium Nitride Materials andMethods Associated with the Same” which is a continuation-in-part ofU.S. patent application Ser. No. 10/879,703, filed Jun. 28, 2004,entitled “Gallium Nitride Materials and Methods Associated with theSame”. All of these applications and their related publications areincorporated herein by reference.

FIELD OF INVENTION

The invention relates generally to gallium nitride materials and, moreparticularly, to gallium nitride material-based structures including astrain-absorbing layer, as well as methods associated with the same.

BACKGROUND OF INVENTION

Gallium nitride materials include gallium nitride (GaN) and its alloyssuch as aluminum gallium nitride (AlGaN), indium gallium nitride(InGaN), and aluminum indium gallium nitride (AlInGaN). These materialsare semiconductor compounds that have a relatively wide, direct bandgapwhich permits highly energetic electronic transitions to occur. Suchelectronic transitions can result in gallium nitride materials having anumber of attractive properties including the ability to efficientlyemit blue light, the ability to transmit signals at high frequency, andothers. Accordingly, gallium nitride materials are being widelyinvestigated in many microelectronic applications such as transistors,field emitters, and optoelectronic devices.

In many applications, gallium nitride materials are grown on asubstrate. However, differences in the properties between galliumnitride materials and substrates can lead to difficulties in growinglayers suitable for many applications. For example, gallium nitride(GaN) has a different thermal expansion coefficient (i.e., thermalexpansion rate) and lattice constants than many substrate materialsincluding sapphire, silicon carbide and silicon. This differences inthermal expansion and lattice constants may lead to formation of defectsincluding misfit dislocations. Misfit dislocations may have a number ofnegative effects including degrading overlying semiconductor materialregions when the dislocations propagate to those regions, creation ofelectronic states within energy bands of those regions that negativelyeffect device performance, and promoting formation of other types ofcrystal defects (e.g., point defects, line defects and planar defects).These effects can negatively impact device performance.

SUMMARY OF INVENTION

The invention provides semiconductor structures including structuresthat comprise a gallium nitride material region and a strain-absorbinglayer, as well as methods associated with the same.

In one embodiment, a semiconductor structure is provided. The structurecomprises a silicon substrate having a top surface; and, an amorphoussilicon nitride-based material layer covering a majority of the topsurface of the substrate. A nitride-based material overlying layer isformed on the silicon nitride-based material layer.

In another embodiment, a semiconductor structure is provided. Thestructure comprises a silicon substrate including a top surface; and, asilicon nitride-based material layer having a thickness of less than 100Angstroms and covering a majority of the top surface of the substrate. Asingle crystal aluminum nitride-based material overlying layer is formedon the silicon nitride-based material layer.

In another embodiment, a semiconductor structure is provided. Thestructure comprises a silicon substrate including a top surface; and, anamorphous silicon nitride-based material layer covering substantiallythe entire top surface of the silicon substrate and having a thicknessof less than 100 Angstroms. A compositionally-graded transition layer isformed on the amorphous silicon nitride-based material layer. A galliumnitride material region is formed on the transition layer.

In another embodiment, a semiconductor structure is provided. Thestructure comprises a semiconductor material region; and, astrain-absorbing layer formed on the semiconductor material region. Anitride-based material layer is formed directly on the strain-absorbinglayer, wherein the misfit dislocation density in the nitride-basedmaterial layer is less than about 10¹⁰ defects/cm².

In another embodiment, a method of forming a semiconductor structure isprovided. The method comprises providing a silicon substrate in areaction chamber; and, introducing a nitrogen source into the reactionchamber to form an amorphous silicon nitride-based material layer. Themethod further comprises introducing a second source into the reactionchamber to form a nitride-based material overlying layer on the siliconnitride-based material layer.

In another embodiment, a method of forming a semiconductor structure isprovided. The method comprises providing a silicon substrate in areaction chamber, introducing a nitrogen source into the reactionchamber to form a silicon nitride-based material layer having athickness of less than 100 Angstroms in a CVD process. The methodfurther comprises introducing a second source into the reaction chamberto form a single crystal nitride-based material overlying layer on thesilicon nitride-based material layer.

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings. Theaccompanying figures are schematic and are not intended to be drawn toscale. In the figures, each identical, or substantially similarcomponent that is illustrated in various figures is represented by asingle numeral or notation. For purposes of clarity, not every componentis labeled in every figure. Nor is every component of each embodiment ofthe invention shown where illustration is not necessary to allow thoseof ordinary skill in the art to understand the invention. All patentapplications and patents incorporated herein by reference areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a gallium nitride material-based semiconductorstructure including a strain-absorbing layer according to one embodimentof the present invention.

FIG. 2 illustrates a gallium nitride material-based semiconductorstructure including a strain-absorbing layer according to anotherembodiment of the present invention.

FIG. 3 illustrates a gallium nitride material-based semiconductorstructure including a strain-absorbing layer formed between layerswithin the structure according to another embodiment of the presentinvention.

FIG. 4 schematically illustrates a FET device including astrain-absorbing layer according to another embodiment of the invention.

FIG. 5 is a copy of a STEM (scanning transmission electron microscope)image that illustrates the presence of a silicon nitridestrain-absorbing layer between an aluminum nitride layer and a siliconsubstrate as described in Example 1.

FIGS. 6 and 7 are copies of high-resolution TEM images that illustratesthe presence of a silicon nitride strain-absorbing layer between analuminum nitride layer and a silicon substrate as described in Example1.

FIG. 8 is a copy of an image published in R. Liu, et. al., Applied Phys.Lett. 83(5), 860 (2003) that illustrates an aluminum nitride layer andsilicon substrate interface without the presence of a silicon nitridestrain-absorbing layer as described in the Comparative Example.

DETAILED DESCRIPTION

The invention provides semiconductor structures including a galliumnitride material region and methods associated with such structures. Thesemiconductor structures can include a strain-absorbing layer formedwithin the structure. The strain-absorbing layer may be formed betweenthe substrate (e.g., a silicon substrate) and an overlying layer, orbetween layers within the structure. As described further below, it maybe preferable for the strain-absorbing layer to be very thin, have anamorphous structure and be formed of a silicon nitride-based material.The strain-absorbing layer may reduce the number of misfit dislocationsformed in the overlying layer (e.g., a nitride-based material layer)which may limit formation of other types of defects in other overlyinglayers (e.g., gallium nitride material region), amongst otheradvantages. Thus, the presence of the strain-absorbing layer may improvethe quality of the gallium nitride material region which can lead toimproved device performance.

FIG. 1 illustrates a semiconductor structure 10 according to oneembodiment of the invention. In the illustrative embodiment, thesemiconductor structure includes a strain-absorbing layer 12 formedbetween a substrate 14 and an overlying layer 15. As shown, thestructure includes a transition layer 16 formed on the overlying layerand a gallium nitride material region 18 formed on the transition layer.As described further below, the composition, thickness and crystalstructure of the strain-absorbing layer may contribute to reducing thenumber of misfit dislocations in the overlying layer which may decreasedefect formation in the gallium nitride material region. This increasesthe quality of the gallium nitride material region and improves deviceperformance. Semiconductor structures of the invention may form thebasis of a number of semiconductor devices including transistors (e.g.,FET), Schottky diodes, light emitting diodes and laser diodes, amongstothers.

When a layer is referred to as being “on” or “over” another layer orsubstrate, it can be directly on the layer or substrate, or anintervening layer also may be present. A layer that is “directly on”another layer or substrate means that no intervening layer is present.It should also be understood that when a layer is referred to as being“on” or “over” another layer or substrate, it may cover the entire layeror substrate, or a portion of the layer or substrate.

The strain-absorbing layer helps absorb strain that arises due tolattice differences between the crystal structure of the substrate andthe crystal structure of overlying layer 15 (e.g., when overlying layer15 is formed of an aluminum nitride-based or gallium nitride material).In the absence of the strain-absorbing layer, this strain is typicallyaccommodated by the generation of misfit dislocations in overlying layer15 at the interface with the substrate. Thus, by providing analternative mechanism for accommodating stress, the presence of thestrain-absorbing layer may reduce the generation of misfit dislocations.

Furthermore, the strain-absorbing layer can help absorb strain thatarises due to differences in the thermal expansion rate of the substrateas compared to the thermal expansion rate of the overlying layer and/orthe gallium nitride material region. Such differences can lead toformation of misfit dislocations at the overlying layer/substrateinterface, or cracking in either the overlying layer and/or galliumnitride material region. As described further below, transition layer 16also helps absorb this thermally-induced strain.

In certain preferred embodiments, strain-absorbing layer 12 is formed ofa silicon nitride-based material. Silicon nitride-based materialsinclude any silicon nitride-based compound (e.g., Si_(x)N_(y), such asSiN and Si₃N₄, SiCN, amongst others) including non-stoichiometricsilicon nitride-based compounds. In some embodiments, a SiNstrain-absorbing layer may be preferred. Silicon nitride material-basedstrain-absorbing layers may be particularly preferred when formeddirectly on a silicon substrate, as described further below.

It should also be understood that it is possible for thestrain-absorbing layer to be formed of other types of materialsaccording to other embodiments of the invention. Though all of theadvantages associated with silicon nitride-based materials may not beachieved in these embodiments.

In some embodiments, it is preferable for the strain-absorbing layer tohave an amorphous (i.e., non-crystalline) crystal structure. Amorphousstrain-absorbing layers are particularly effective in accommodatingstrain and, thus, reducing the generation of misfit dislocations. Itshould be understood that amorphous strain-absorbing layers, asdescribed herein, are entirely amorphous being free of regions thatexhibit a crystalline structure. In some preferred embodiments,amorphous strain-absorbing layers are formed of a silicon nitride-basedmaterial as described above.

However, it should be understood that in certain embodiments of theinvention the strain-absorbing layer may have a single crystalstructure, a poly-crystalline structure, or a structure that includescrystalline and amorphous regions.

In some embodiments, it is preferred for the strain-absorbing layer tobe very thin, particularly when formed of amorphous and/or siliconnitride-based materials. It has been discovered that very thinstrain-absorbing layers (e.g., silicon nitride-based strain-absorbinglayers) may enable formation of overlying layer(s) having an epitaxialrelationship with the substrate, while also being effective in reducingthe number of misfit dislocations. In certain cases (e.g., when thestrain-absorbing layer is amorphous), it is desirable for thestrain-absorbing layer to have a thickness that is large enough toaccommodate sufficient strain associated with lattice and thermalexpansion differences between the substrate and overlying layer 15 toreduce generation of misfit dislocations. In these cases, it may also bedesirable for the strain-absorbing layer to be thin enough so that theoverlying layer has an epitaxial relationship with the substrate. Thiscan be advantageous for formation of a high quality, single crystalgallium nitride material region. If the strain-absorbing layer is toothick, then the overlying layer is not formed with epitaxialrelationship with the substrate.

In some embodiments, the strain-absorbing layer has a thickness of lessthan about 100 Angstroms which, in these embodiments, can allow theepitaxial relationship between the substrate and the overlying layer. Insome embodiments, it may be preferable for the strain-absorbing layer tohave a thickness of less than about 50 Angstroms to allow for theepitaxial relationship.

The strain-absorbing layer may have a thickness of greater than about 10Angstroms which, in these embodiments, is sufficient to accommodatestrain (e.g., resulting from lattice and thermal expansion differences)and can facilitate forming a strain-absorbing layer that covers theentire substrate, as described further below. In other embodiments, thestrain-absorbing layer may have a thickness of greater than about 20Angstroms to sufficiently accommodate strain.

Suitable thickness ranges for the strain-absorbing layer include all ofthose defined by the ranges described above (e.g., greater than about 10Angstroms and less than about 100 Angstroms, greater than about 10Angstroms and less than about 50 Angstroms, and the like). Also, thestrain-absorbing layer thickness may be between about 20 Angstroms andabout 70 Angstroms.

It should be understood that suitable thicknesses of thestrain-absorbing layer may depend on a number of factors including thecomposition and crystal structure of the strain-absorbing layer; thecomposition, thickness and crystal structure of the overlying layer; aswell as the composition, thickness, and crystal structure of thesubstrate, amongst other factors. Suitable thicknesses may be determinedby measuring the effect of thickness on misfit dislocation density andother factors (e.g., the ability to deposit an overlying layer having anepitaxial relationship with the substrate, etc.). It is also possiblefor the strain-absorbing layer to have a thickness outside the aboveranges.

In some cases, the thickness of the strain-absorbing layer is relativelyuniform across the entire layer. For example, in these cases, thestrain-absorbing layer may have a thickness uniformity variation of lessthan 25 percent, or less than 10 percent, across the entirestrain-absorbing layer.

As described further below, in some embodiments, the strain-absorbinglayer may be formed by nitridating a top surface region of a siliconsubstrate. That is, the surface region of the substrate may be convertedfrom silicon to a silicon nitride-based material to form thestrain-absorbing layer. It should be understood that, as used herein,such strain-absorbing layers may be referred to as being “formed on thesubstrate”, “formed over the substrate”, “formed directly on thesubstrate” and as “covering the substrate”. Such phrases also refer tostrain-absorbing layers that are formed by depositing a separate layer(e.g., using a separate nitrogen source and silicon source) on the topsurface of the substrate and are not formed by converting a surfaceregion of the substrate.

In the illustrative embodiment, the strain-absorbing layer coverssubstantially the entire top surface of the substrate. This arrangementmay be preferable to minimize the number of misfit dislocations in theoverlying layer. In other embodiments, the strain-absorbing layer maycover a majority of the top surface of the substrate (e.g., greater than50 percent or greater than 75 percent of the top surface area).

Also, in the illustrative embodiment, strain-absorbing layer 12 isformed across the entire area between the substrate and the overlyinglayer. That is, the strain-absorbing layer separates the substrate andthe overlying layer at all points with the strain-absorbing layer beingdirectly on the substrate and the overlying layer being directly on thestrain-absorbing layer. This arrangement may be preferable to minimizethe number of misfit dislocations in the overlying layer. In otherembodiments, the strain-absorbing layer may be formed across a majorityof the area (e.g., greater than 50 percent, or greater than 75 percent)between the substrate and the overlying layer. If the strain-absorbinglayer is not present across the entire (or, at least, the majority ofthe) area between the substrate and the overlying layer, the above-notedadvantages associated with the strain-absorbing layer may not berealized.

The extent that the strain-absorbing layer covers the substrate (and thearea between the overlying layer and the substrate) in the presentinvention may be distinguished from certain prior art techniques inwhich a discontinuous silicon nitride layer is formed (in some cases,inadvertently) between a silicon substrate and an overlying layer.

It should be understood that, in other embodiments, the strain-absorbinglayer may be positioned in other locations such as between two differentlayers (e.g., the embodiment of FIG. 3). In these embodiments, thestrain-absorbing layer may reduce the formation of misfit dislocationsin the layer that overlies the strain-absorbing layer.

As noted above, the presence of the strain-absorbing layeradvantageously results in very low misfit dislocation densities withinthe overlying layer (e.g., at, or very near, an interface between thestrain-absorbing layer and the overlying layer). Misfit dislocationstypically are formed at (or, very near) the interface between twomaterials as a result of incoherency due to differences in atomicstructures of the materials.

In some embodiments of the invention, the misfit dislocation density inthe overlying layer is less than about 10¹⁰ defects/cm²; and, in otherembodiments, less than about 10⁸ defects/cm². Even lower misfitdislocation densities in the overlying layer may be achieved, forexample, less than about 10⁵ defects/cm². In some cases, the presence ofmisfit dislocations may not be readily detectable which generally meansthat the misfit dislocation density is less than about 10² defects/cm².The specific misfit dislocation density depends, in part, on theparticular structure including factors such as the thickness,composition and crystal structure of the strain-absorbing layer; thecomposition, thickness and crystal structure of the overlying layer; aswell as the composition, thickness, and crystal structure of thesubstrate, amongst other factors.

It should be understood that the above-described misfit dislocationdensity ranges may be found in the overlying layer at, or very near(e.g., 20 nm), the interface with the strain-absorbing layer; and, alsomay be found at other regions within the overlying layer.

Misfit dislocation density may be measured using known techniques. Thetechniques generally involve inspection of the atomic structure of asample (e.g., an interface) using high magnification to determine thepresence of misfit dislocations over a representative area. For example,high resolution transmission electron microscopy (TEM) may be used. Onesuitable technique involves counting the number of dislocations over arepresentative area using high resolution-TEM images. The misfitdislocation density is calculated by dividing the number of dislocationsby the area. Typically, the misfit dislocation density is expressed inunits of defects/cm².

It should be understood that, in certain embodiments of the invention,the overlying layer may have misfit dislocation densities greater thanthe above-noted ranges.

The very low misfit dislocation densities achievable in the overlyinglayer in structures of the present invention may lead to a number ofadvantages including reducing defects in the gallium nitride materialregion, as described further below.

It may be preferred for structure 10 to include an overlying layer 15formed of a nitride-based material. Suitable nitride-based materialsinclude, but are not limited to, aluminum nitride-based materials (e.g.,aluminum nitride, aluminum nitride alloys) and gallium nitridebased-materials (e.g., gallium nitride, gallium nitride alloys). Though,in some embodiments, it may be preferred for the overlying layer to havea low gallium concentration or be free of gallium and formed, forexample, of aluminum nitride-based materials. The presence of gallium inthe overlying layer can enhance thermal expansion and lattice mismatchdifferences between the silicon substrate and the overlying layer whichcan lead to cracking, amongst other problems.

In some cases, the overlying layer has a constant composition. In othercases, as described further below, the overlying layer may becompositionally-graded. Suitable compositionally-graded layers aredescribed further below and have been described in commonly-owned U.S.Pat. No. 6,649,287, entitled “Gallium Nitride Materials and Methods”filed on Dec. 14, 2000, which is incorporated herein by reference.

It may be preferable for the overlying layer to have a single crystalstructure. As noted above, in some embodiments, the thickness of thestrain-absorbing layer is controlled so that the overlying layer has anepitaxial relationship with the substrate. It may be advantageous forthe overlying layer to have a single crystal structure because itfacilitates formation of a single crystal, high quality gallium nitridematerial region. In some embodiments, the overlying layer has adifferent crystal structure than the substrate. It should also beunderstood that the overlying layer may not have a single crystalstructure and may be amorphous or polycrystalline, though all of theadvantages associated with the single crystal overlying layers may notbe achieved.

The overlying layer may have any suitable thickness. For example, theoverlying layer may be between about 10 nanometers and 5 microns, thoughother thicknesses are also possible.

In the illustrative embodiment, transition layer 16 is formed directlyon the overlying layer. In certain embodiments, such as when theoverlying layer has a constant composition, it may be preferred for thetransition layer to be formed of a compositionally-graded material(e.g., a compositionally-graded nitride-based material). Suitablecompositionally-graded layers have been described in commonly-owned U.S.Pat. No. 6,649,287 which is incorporated by reference above.Compositionally-graded transition layers have a composition that isvaried across at least a portion of the layer. Compositionally-gradedtransition layers are particularly effective in reducing crack formationin gallium nitride material regions formed on the transition layer bylowering thermal stresses that result from differences in thermalexpansion rates between the gallium nitride material and the substrate(e.g., silicon).

According to one set of embodiments, the transition layer iscompositionally-graded and formed of an alloy of gallium nitride such asAl_(x)In_(y)Ga_((1-x-y))N, Al_(x)Ga_((1-x))N, and In_(y)Ga_((1-y))N. Inthese embodiments, the concentration of at least one of the elements(e.g., Ga, Al, In) of the alloy is varied across at least a portion ofthe thickness of the transition layer. When transition layer 16 has anAl_(x)In_(y)Ga_((1-x-y))N composition, x and/or y may be varied. Whenthe transition layer has a Al_(x)Ga_((1-x))N composition, x may bevaried. When the transition layer has a In_(y)Ga_((1-y))N composition, ymay be varied.

In certain preferred embodiments, it is desirable for the transitionlayer to have a low gallium concentration at a back surface which isgraded to a high gallium concentration at a front surface. It has beenfound that such transition layers are particularly effective inrelieving internal stresses within gallium nitride material region 18.For example, the transition layer may have a composition ofAl_(x)Ga_((1-x))N, where x is decreased from the back surface to thefront surface of the transition layer (e.g., x is decreased from a valueof 1 at the back surface of the transition layer to a value of 0 at thefront surface of the transition layer).

In one preferred embodiment, structure 10 includes an aluminum nitrideoverlying layer 15 and a compositionally-graded transition layer 16. Thecompositionally-graded transition layer may have a composition ofAl_(x)Ga_((1-x))N, where x is graded from a value of 1 at the backsurface of the transition layer to a value of 0 at the front surface ofthe transition layer. The composition of the transition layer, forexample, may be graded discontinuously (e.g., step-wise) orcontinuously. One discontinuous grade may include steps of AlN,Al_(0.6)Ga_(0.4)N and Al_(0.3)Ga_(0.7)N proceeding in a direction towardthe gallium nitride material region.

It should be understood that, in other cases, transition layer 16 mayhave a constant composition and may not be compositionally-graded (e.g.,when the overlying layer is compositionally-graded). It should also beunderstood that in some embodiments of the invention, as shown in FIG.2, a separate transition layer is not present between the overlyinglayer and the gallium nitride material region. In the illustrativeembodiment of FIG. 2, structure 20 includes overlying layer 15 formeddirectly on top of strain-absorbing layer 12 and gallium nitridematerial region 18 formed directly on the overlying layer. In thisembodiment, it may be preferable for the overlying layer to becompositionally-graded as described above.

The overlying layer and/or transition layer are typically (though notalways) not part of the active region of the device. As described above,the overlying layer and/or transition layer may be formed to facilitatedeposition of gallium nitride material region 18. However, in somecases, the overlying layer and/or transition layer may have otherfunctions including functioning as a heat spreading layer that helpsremove heat from active regions of the semiconductor structure duringoperation of a device. For example, such transition layers that functionas heat spreading layers have been described in commonly-owned,co-pending U.S. patent application Ser. No. 09/792,409 entitled “GalliumNitride Materials Including Thermally-Conductive Regions,” filed Feb.23, 2001, which is incorporated herein by reference.

Active regions of the device may be formed in gallium nitride materialregion 18. Gallium nitride material region 18 comprises at least onegallium nitride material layer. As used herein, the phrase “galliumnitride material” refers to gallium nitride (GaN) and any of its alloys,such as aluminum gallium nitride (Al_(x)Ga_((1-x))N), indium galliumnitride (In_(y)Ga_((1-y))N), aluminum indium gallium nitride(Al_(x)In_(y)Ga_((1-x-y))N), gallium arsenide phosporide nitride(GaAs_(a)P_(b) N_((1-a-b))), aluminum indium gallium arsenide phosporidenitride (Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b) N_((1-a-b))), amongstothers. Typically, when present, arsenic and/or phosphorous are at lowconcentrations (i.e., less than 5 weight percent). In certain preferredembodiments, the gallium nitride material has a high concentration ofgallium and includes little or no amounts of aluminum and/or indium. Inhigh gallium concentration embodiments, the sum of (x+y) may be lessthan 0.4, less than 0.2, less than 0.1, or even less. In some cases, itis preferable for the gallium nitride material layer to have acomposition of GaN (i.e., x+y=0). Gallium nitride materials may be dopedn-type or p-type, or may be intrinsic. Suitable gallium nitridematerials have been described in U.S. Pat. No. 6,649,287, incorporatedby reference above.

In some cases, gallium nitride material region 18 includes only onegallium nitride material layer. In other cases, gallium nitride materialregion 18 includes more than one gallium nitride material layer. Forexample, the gallium nitride material region may include multiple layers(e.g., 18 a, 18 b, 18 c) as shown in FIG. 4. In certain embodiments, itmay be preferable for the gallium nitride material of layer 18 b to havean aluminum concentration that is greater than the aluminumconcentration of the gallium nitride material of layer 18 a. Forexample, the value of x in the gallium nitride material of layer 18 b(with reference to any of the gallium nitride materials described above)may have a value that is between 0.05 and 1.0 greater than the value ofx in the gallium nitride material of layer 18 a, or between 0.05 and 0.5greater than the value of x in the gallium nitride material of layer 18a. For example, layer 18 b may be formed of Al_(0.26)Ga_(0.74)N, whilelayer 18 a is formed of GaN. This difference in aluminum concentrationmay lead to formation of a highly conductive region at the interface ofthe layers 18 b, 18 a (i.e., a 2-D electron gas region). In theillustrative embodiment, layer 18 c may be formed of GaN.

Suitable gallium nitride material layer arrangements have beendescribed, for example, in commonly-owned, co-pending U.S. patentapplication Ser. No. 10/740,376 entitled “Gallium Nitride MaterialDevices Including an Electrode-Defining Layer and Methods of Forming theSame,” filed Dec. 17, 2003 which is incorporated herein by reference.

Gallium nitride material region 18 also may include one or more layersthat do not have a gallium nitride material composition such as otherIII-V compounds or alloys, oxide layers, and metallic layers.

Gallium nitride material region 18 is of high enough quality so as topermit the formation of devices therein. As noted above, the presence ofthe strain-absorbing layer may reduce the misfit dislocation density inthe overlying layer which can reduce formation of defects in the galliumnitride material region. For example, the generation of point defects,line defects, and planar defects may be reduced. By limiting defectgeneration in the gallium nitride material region, device performancecan be improved. The low misfit dislocation densities can also limitcreation of electronic states within energy bands of the gallium nitridematerial regions which also negatively effect device performance.

Preferably, gallium nitride material region 18 also has a low cracklevel. As described above, the transition layer (particularly whencompositionally-graded) and/or Is overlying layer may reduce crackformation. Gallium nitride materials having low crack levels have beendescribed in U.S. Pat. No. 6,649,287 incorporated by reference above. Insome cases, the gallium nitride material region has a crack level ofless than 0.005 μm/μm². In some cases, the gallium nitride materialregion has a very low crack level of less than 0.001 μm/μm². In certaincases, it may be preferable for the gallium nitride material region tobe substantially crack-free as defined by a crack level of less than0.0001 μm/μm².

In certain cases, gallium nitride material region 18 includes a layer(or layers) which have a single crystal (i.e., monocrystalline)structure. In some cases, the gallium nitride material region includesone or more layers having a Wurtzite (hexagonal) structure.

The thickness of gallium nitride material region 18 and the number ofdifferent layers are dictated, at least in part, by the requirements ofthe specific device. At a minimum, the thickness of the gallium nitridematerial region is sufficient to permit formation of the desiredstructure or device. The gallium nitride material region generally has athickness of greater than 0.1 micron, though not always. In other cases,gallium nitride material region 18 has a thickness of greater than 0.5micron, greater than 2.0 microns, or even greater than 5.0 microns.

As described above, in certain preferred embodiments, substrate 14 is asilicon substrate. As used herein, a silicon substrate refers to anysubstrate that includes a silicon surface. Examples of suitable siliconsubstrates include substrates that are composed entirely of silicon(e.g., bulk silicon wafers), silicon-on-insulator (SOI) substrates,silicon-on-sapphire substrate (SOS), and SIMOX substrates, amongstothers. Suitable silicon substrates also include substrates that have asilicon wafer bonded to another material such as diamond, AlN, or otherpolycrystalline materials. Silicon substrates having differentcrystallographic orientations may be used, though single crystal siliconsubstrates are preferred. In some cases, silicon (111) substrates arepreferred. In other cases, silicon (100) substrates are preferred.

It should be understood that other types of substrates may also be usedincluding sapphire, silicon carbide, indium phosphide, silicongermanium, gallium arsenide, gallium nitride, aluminum nitride, or otherIII-V compound substrates. However, in embodiments that do not usesilicon substrates, all of the advantages associated with siliconsubstrates may not be achieved. In some embodiments, it may bepreferable to use non-nitride material-based substrates such as silicon,sapphire, silicon carbide, indium phosphide, silicon germanium andgallium arsenide.

Substrate 14 may have any suitable dimensions and its particulardimensions are dictated by the application. Suitable diameters include,but are not limited to, about 2 inches (50 mm), 4 inches (100 mm), 6inches (150 mm), and 8 inches (200 mm). Advantageously, thestrain-absorbing layer may be used to form a high quality galliumnitride material region on substrates (e.g., silicon substrates) over avariety of thicknesses. In some cases, it may be preferable for thesubstrate to be relatively thick, such as greater than about 125 micron(e.g., between about 125 micron and about 800 micron, or between about400 micron and 800 micron). Relatively thick substrates may be easy toobtain, process, and can resist bending which can occur, in some cases,in thinner substrates. In other embodiments, thinner substrates (e.g.,less than 125 microns) are used, though these embodiments may not havethe advantages associated with thicker substrates, but can have otheradvantages including facilitating processing and/or reducing the numberof processing steps. In some processes, the substrate initially isrelatively thick (e.g., between about 200 microns and 800 microns) andthen is thinned during a later processing step (e.g., to less than 150microns).

In some preferred embodiments, the substrate is substantially planar inthe final device or structure. Substantially planar substrates may bedistinguished from substrates that are textured and/or have trenchesformed therein (e.g., as in U.S. Pat. No. 6,265,289). As shown, thelayers/regions of the device (e.g., strain-absorbing layer, overlyinglayer, transition layer, gallium nitride material region) may also besubstantially planar in the final device or structure. As describedfurther below, such layers/regions may be grown in vertical (e.g.,non-lateral) growth processes. Planar substrates and layers/regions canbe advantageous in some embodiments, for example, to simplifyprocessing. Though it should be understood that, in some embodiments ofthe invention, lateral growth processes may be used as described furtherbelow.

FIG. 3 illustrates a semiconductor structure 22 according to anotherembodiment of the invention. In this embodiment, strain-absorbing layer12 is formed between layers within the structure, and is not formeddirectly on the substrate. For example, the strain-absorbing layer maybe formed between an underlying layer 24 and an overlying layer 15. Inthis embodiment, the strain-absorbing layer may reduce the formation ofmisfit dislocations in overlying layer 15 as described above inconnection with the embodiments of FIG. 1.

Underlying layer 24 may be formed of a variety of semiconductormaterials. In some embodiments, the underlying layer is formed of anitride-based material. Suitable nitride-based materials include, butare not limited to, aluminum nitride-based materials (e.g., aluminumnitride, aluminum nitride alloys) and gallium nitride materials. In someembodiments, it may be preferred for the underlying material to have adifferent composition than the overlying material. The underlying layermay also have a different crystal structure than the overlying layer.

In other embodiments, the underlying material may be formed ofnon-nitride based materials.

The semiconductor structures illustrated in FIGS. 1-3 may form the basisof a variety of semiconductor devices. Suitable devices include, but arenot limited to, transistors (e.g., FETs) as well as light-emittingdevices including LEDs and laser diodes. The devices have active regionsthat are typically, at least in part, within the gallium nitridematerial region. Also, the devices include a variety of other functionallayers and/or features (e.g., electrodes).

The strain-absorbing layer may be included in structures and devicesdescribed in commonly-owned, co-pending U.S. patent application Ser. No.10/740,376, incorporated by reference above. For example, FIG. 4schematically illustrates a FET device 30 according to one embodiment ofthe invention which is similar to a FET device described in U.S. patentapplication Ser. No. 10/740,376 except device 30 includesstrain-absorbing layer 12. Device 30 includes a source electrode 34, adrain electrode 36 and a gate electrode 38 formed on gallium nitridematerial region 18 (which includes a first layer 18 b and a second layer18 a). The device also includes an electrode defining layer 40 which, asshown, is a passivating layer that protects and passivates the surfaceof the gallium nitride material region. A via 42 is formed within theelectrode defining layer in which the gate electrode is, in part,formed. Strain-absorbing layer 12 is formed directly on the substrateand overlying layer 15 is formed directly on the strain-absorbing layer.In some embodiments, the overlying layer is compositionally-graded. Insome embodiments, the overlying layer may have a constant composition(e.g., aluminum nitride or an aluminum nitride alloy) and acompositionally-graded transition layer is formed on thestrain-absorbing layer.

The strain-absorbing layer may also be included in structures anddevices described in U.S. Pat. No. 6,649,287 which is incorporatedherein by reference above.

The strain-absorbing layer may also be included in structures anddevices described in commonly-owned U.S. Pat. No. 6,611,002 entitled“Gallium Nitride Material Devices and Methods Including Backside Vias”which is incorporated herein by reference.

It should be understood that other structures and devices that use thestrain-absorbing layer may be within the scope of the present inventionincluding structures and devices that are not specifically describedherein. Other structures may include other layers and/or features,amongst other differences.

Semiconductor structure 10 may be manufactured using known semiconductorprocessing techniques. In embodiments in which the strain-absorbinglayer is a silicon nitride-based material (e.g., amorphous SiN), thestrain-absorbing layer may be formed by nitridating a top surface of thesilicon substrate as noted above. In a nitridation process, nitrogenreacts with a top surface region of the silicon substrate to form asilicon nitride-based layer. The top surface may be nitridated byexposing the silicon substrate to a gaseous source of nitrogen atelevated temperatures in a CVD process such as an MOCVD process. Forexample, ammonia may be introduced into a reaction chamber in which asilicon substrate is positioned. The temperature in the reaction chambermay be greater than 950° C., such as between about 1000° C. and about1400° C., or, more typically, between about 1000° C. and about 1100° C.The pressure may be greater than about 1 torr, such as between about 1torr and about 10³ torr, or, more typically, between about 20 torr andabout 40 torr (in some cases, about 30 torr). The reaction betweennitrogen and the silicon substrate is allowed to proceed for a reactiontime (e.g., less than 30 seconds) selected to produce a layer having adesired thickness.

It should be understood that other processes may be used to form siliconnitride-based strain-absorbing layers including processes (e.g., CVDprocesses) that use separate nitrogen and silicon sources. Also, whenthe strain-absorbing layer is formed of another type of material(non-silicon nitride-based material), other deposition processes knownin the art are used.

In some embodiments, the strain-absorbing layer may be formed in-situwith the overlying layer (and, in some cases, subsequent layers) of thestructure. That is, the strain-absorbing layer may be formed during thesame deposition process (e.g., MOCVD process) as the overlying layer(and, in some cases, subsequent layers). In MOCVD processes that grow asilicon nitride-based material strain-absorbing layer by introducing anitrogen source (e.g., ammonia) into a reaction chamber as describedabove, a second source gas may be introduced into the chamber after aselected time delay after the nitrogen source. The second source reactswith the nitrogen source to form the overlying layer, thus, endinggrowth of the strain-absorbing layer. For example, when the overlyinglayer is formed of aluminum nitride, an aluminum source (e.g.,trimethylaluminum) is introduced into the chamber at a selected timeafter the nitrogen source (e.g., ammonia). The time delay is selected sothat the strain-absorbing layer grows to a desired thickness. Thereaction between the second source (e.g., aluminum source) and thenitrogen source is allowed to proceed for a sufficient time to producethe overlying layer. To simplify the process amongst other reasons, insome embodiments, it is preferred that the overlying layer(s) and thestrain-absorbing layer are formed in the same MOCVD process which mayhave the same reaction conditions (e.g., temperature, pressure). Growingthe strain-absorbing and overlying layers with an MOCVD process isparticularly preferred in the present invention in order to produce highquality gallium nitride.

When the overlying layer has a single crystal structure, the reactionconditions are selected appropriately. For example, the reactiontemperature may be greater than 950° C., such as between about 1000° C.and about 1400° C., or, more typically, between about 1000° C. and about1100° C. In some cases, lower growth temperatures may be used includingtemperatures between about 500° C. and about 600° C., though highertemperatures may be preferred in order to produce higher qualitymaterial. The pressure may be greater than about 1 torr, such as betweenabout 1 torr and about 10³ torr; or, more typically, between about 20torr and about 40 torr (in some cases, about 30 torr). It should also beunderstood that the strain-absorbing layer may be formed in a separateprocess than the overlying layer and subsequent layers. For example, thestrain-absorbing layer may be formed on the substrate in a firstprocess. Then, at a later time, the overlying layers may be formed onthe strain-absorbing layer in a second process.

In the processes described above, the overlying layer is grown in avertical growth process. That is, the overlying layer is grown in avertical direction with respect to the strain-absorbing layer. Theability to vertically grow the strain-absorbing layer having low misfitdislocation densities may be advantageous as compared to lateral growthprocesses which may be more complicated.

Transition layer 16 and gallium nitride material region 18 may also begrown in the same deposition step as the overlying layer and thestrain-absorbing layer. In such processes, suitable sources areintroduced into the reaction chamber at appropriate times. SuitableMOCVD processes to form compositionally-graded transition layers andgallium nitride material region over a silicon substrate have beendescribed in U.S. Pat. No. 6,649,287 incorporated by reference above.When gallium nitride material region 18 has different layers, in somecases, it is preferable to use a single deposition step to form theentire region 18. When using the single deposition step, the processingparameters may be suitably changed at the appropriate time to form thedifferent layers.

It should also be understood that the transition layer and the galliumnitride material region may be grown separately from thestrain-absorbing layer and overlying layer. The gallium nitride materialregion and transition layer may be grown in a vertical growth process.That is, these regions are grown in a vertical direction with respect tounderlying layers. The ability to vertically grow the gallium nitridematerial region having low misfit dislocation densities may beadvantageous as compared to lateral growth processes which may be morecomplicated.

However, in other embodiments of the invention (not shown), it ispossible to grow, at least a portion of, gallium nitride material region18 using a lateral epitaxial overgrowth (LEO) technique that involvesgrowing an underlying gallium nitride layer through mask openings andthen laterally over the mask to form the gallium nitride materialregion, for example, as described in U.S. Pat. No. 6,051,849.

In other embodiments of the invention (not shown), it is possible togrow the gallium nitride material region 18 using a pendeoepitaxialtechnique that involves growing sidewalls of gallium nitride materialposts into trenches until growth from adjacent sidewalls coalesces toform a gallium nitride material region, for example, as described inU.S. Pat. No. 6,265,289. In these lateral growth techniques, galliumnitride material regions with very low defect densities are achievable.For example, at least a portion of the gallium nitride material regionmay have a defect density of less than about 10⁵ defects/cm².

Commonly-owned, co-pending U.S. patent application Ser. No. 10/740,376,incorporated by reference above, further describes techniques used togrow other layers and features shown in the embodiment of FIG. 4.

It should also be understood that other processes may be used to formstructures and devices of the present invention as known to those ofordinary skill in the art.

The following examples are meant to be illustrative and is not limiting.

EXAMPLE 1

This example illustrates the formation of a silicon nitride-basedmaterial strain-absorbing layer on a silicon substrate according to oneembodiment of the present invention.

A 100 mm silicon substrate was placed in a reaction chamber. Ammonia gaswas introduced into the chamber as a nitrogen source. The temperaturewas maintained at 1030° C. and the pressure at about 30 torr. A layer ofamorphous silicon nitride (SiN) was formed.

About 6 seconds after the introduction of ammonia, TMA was introducedinto the chamber as an aluminum source. The temperature and pressurewere respectively maintained at 1030° C. and about 30 torr. Growthproceeded for 30 minutes.

FIGS. 5-7 are copies of micrograph images that illustrate the resultingstructure. FIG. 5 is a copy of a STEM (scanning transmission electronmicroscope) image. FIGS. 6 and 7 are copies of high-resolution TEMimages. The images show the presence of an amorphous silicon nitridestrain-absorbing layer formed between a single crystal aluminum nitridelayer and a single crystal silicon substrate. In particular, thehigh-resolution TEM images show the crystal structures of the resultinglayers and substrate. The images show that the crystal structure of thesilicon nitride layer is amorphous, the crystal structure of the siliconsubstrate is cubic and the crystal structure of the aluminum nitride ishexagonal. The aluminum nitride layer has an epitaxial relationship withthe substrate.

The images also show the absence of misfit dislocations at (or near) theinterface of the amorphous silicon nitride strain-absorbing layer andthe aluminum nitride layer.

This example establishes that strain-absorbing layers of the presentinvention may be used to limit misfit dislocation density.

COMPARATIVE EXAMPLE

This example illustrates the presence of misfit dislocations in analuminum nitride layer formed directly on a silicon substrate in theabsence of a strain-absorbing layer of the present invention.

FIG. 8 is a copy of an image published in R. Liu, et. al., Applied Phys.Lett. 83(5), 860 (2003). The image illustrates an aluminum nitride layerformed directly on a silicon substrate, without the presence of asilicon nitride strain-absorbing layer, following procedures describedin the article. Misfit dislocations are indicated by “^(⊥)”. Theinterface coherence is indicated by solid lines that connect {111}_(Si)and {1-100}_(AlN) lattice planes.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A method of forming a semiconductor structure comprising: providing asubstrate; forming an amorphous silicon nitride-based material layer onthe substrate; and forming a nitride-based material overlying layer onthe silicon nitride-based material layer.
 2. The method of claim 1,comprising providing the substrate in a reaction chamber and furthercomprising introducing at least a nitrogen source into the reactionchamber to form the amorphous silicon nitride-based material layer. 3.The method of claim 2, further comprising introducing a second sourceinto the reaction chamber to form a nitride-based material overlyinglayer on the silicon nitride-based material layer.
 4. The method ofclaim 2, further comprising introducing a silicon source into thereaction chamber to react with the nitrogen source to form the amorphoussilicon nitride-based material layer.
 5. The method of claim 2,comprising introducing a nitrogen source into the reaction chamber toform an amorphous silicon nitride-based material layer in a MOCVDprocess.
 6. The method of claim 2, wherein the temperature in thereaction chamber is between about 1000° C. and about 1400° C. when thenitrogen source is introduced into the chamber.
 7. The method of claim2, wherein the pressure in the reaction chamber is between about 1 torrand about 103 torr when the nitrogen source is introduced into thechamber.
 8. The method of claim 2, wherein the pressure in the reactionchamber is between about 20 torr and about 40 torr when the nitrogensource is introduced into the chamber.
 9. The method of claim 2, whereinthe nitrogen source is ammonia.
 10. The method of claim 2, wherein thesubstrate is silicon and the nitrogen source reacts with the siliconsubstrate to form a silicon nitride-based material layer.
 11. The methodof claim 3, wherein the second source comprises an aluminum source. 12.The method of claim 3, wherein the aluminum source reacts with thenitrogen source to form a layer of aluminum nitride.
 13. The method ofclaim 1, wherein the substrate is silicon.
 14. The method of claim 1,wherein the substrate is silicon carbide.
 15. A method of forming asemiconductor structure comprising: providing a substrate in a reactionchamber; introducing at least a nitrogen source into the reactionchamber to form a silicon nitride-based material layer having athickness of less than 100 Angstroms in a CVD process; and introducing asecond source into the reaction chamber to form a single crystalnitride-based material overlying layer on the silicon nitride-basedmaterial layer.